This magnetic field viewing kit is mess free and stays clear! The magnets are rubber coated to prevent scratching on the outside of the tray. The steel filings are sealed in a scratch-resistant, transparent viewing tray and are great for learning about magnetic fields. For ages 8 and up. Contents: 1 rubber disc magnet: .75" 1 rubber bar magnet: 1" 1 Filing-filled tray: 3.44" diameter For ages 8 and up